In the manufacture of commonly known NTCR thermistors, the materials are typically bulk sintered bodies of mixed oxides of metals such as Bi, Ru, Fe, Ni, Co, Mn, W, Mo and the like, or thick film compositions involving these or similar materials sintered together or in a glass binder. They are provided with two metallic electrical contacts, typically alloys of noble metals, fired on at high temperatures. These devices have high sensitivities, limiting the temperature range of a single device. Since the bulk thermistors are "stand alone" units, and the thick films must be made with techniques such as silk screening, their size and utility for integrating with other devices is limited. They must also be fired at high temperatures (greater than 500.degree. C.), which limits their possible integration with other devices, such as silicon integrated circuits.
Thin film thermistors are also known, such as doped silicon carbide and silicon semiconductor resistors, gold particles in germanium, and platinum particles in alumina, formed on one side of a substrate and provided with two electrical contacts. See, U.S. Pat. No. 4,359,372 to Nagai et al, the entire content of which is expressly incorporated hereinto by reference. The doped semiconductor thermistors are conventional positive temperature coefficient (PTCR) devices with limited adjustability of TCR, and the metal precipitates in insulating matrices are either difficult to make controllably or are unstable at temperatures of 20.degree. C. to 300.degree. C. (See in this regard, U.S. Pat. Nos. 5,158,933 to Holtz et al and 4,370,640 to Dynes et al.) Semiconductor thermistors are also prone to magnetic field-induced errors.
A zirconium nitride thin film thermistor is also known from published Japanese Patent Application (Kokai) No. 63-224201 to Yotsuya et al. More particularly, the thermistor disclosed in Yotsuya et al is a system which includes zirconium nitride as an electrical conductor and "excess nitrogen" as an electrical insulator or defect-causing additive.
The type of system incorporating mixtures of conducting and insulating phases and exhibiting more-or-less logarithmic, NTCR variation of resistance with temperature are variously described as "percolation" or "hopping" conductivity systems. The insulating or defect-causing phases do not "dope" the conductor in the sense of a semiconductor, but instead interfere with the conduction of electrical current in the metal by introducing barriers to conduction, which must be circumvented, surmounted or tunneled through. As more insulating phase is added to the conducting phase, the resistivity increases. At the same time, the temperature coefficient of resistance (TCR) decreases from the positive value of the pure metal. With further addition of the insulating or defect-causing phase, the TCR passes through zero and increases in negative value. In most systems, such as cermets (e.g., platinum particles dispersed in alumina), a transition to an insulating state is eventually reached. In others, such as the Zr/excess nitrogen system disclosed in Yotsuya et al cited above, the insulating stage is never reached.
The type of material disclosed in Yotsuya et al is also very wide range (millikelvins to room temperature (295.+-.5K) and very resistant to magnetic field-induced errors, even at liquid helium temperatures (4.2K); for instance, less than 1% of temperature error up to 5 Tesla magnetic fields. There are, however, several disadvantages associated with the type of material disclosed in Yotsuya et al, such as:
1. The "excess nitrogen" insulating phase is unstable, even at temperatures as low as 375K, because the "excess nitrogen" is only weakly bound and causes its insulating effect with defects and included gas that are metastable. PA1 2. There is a limit to the amount of "excess nitrogen" the films can take up, and the range of resistances and TCR's obtainable are limited. PA1 3. Other metals which have conducting nitrides do not uniformly take up "excess nitrogen" in the same way zirconium does, and therefore what desirable properties those metals may have are not available for exploitation.
The present invention is embodied in novel metal oxy-nitride alloys which form electrical film resistors and which can be "engineered" so as to exhibit the desired TCR and which address many of the disadvantages of conventional film thermistors described above.
Broadly, the present invention provides film resistors which are formed of an alloy of both an electrically insulating oxide and an electrically conducting nitride of at least one metal. During manufacture of the thin film resistance materials according to this invention, reactive gases of an oxygen-containing gas and nitrogen gas are introduced into a reaction chamber (preferably as a mixture) in the presence of an inert gas and a metal or a metal composite (e.g., pressed powder) target capable of forming both oxides and nitrides. The ratio of the oxygen-containing gas to nitrogen gas and/or the total volume flow rate are selectively controlled so as to obtain a desired amount of metal oxide to metal nitride in the oxy-nitride metal film that is formed in the reaction chamber. By varying the amount of oxygen-containing gas in the reaction gas and/or flow rate of the reaction gas with all remaining parameters constant, the amount of metal oxide in the metal film alloy may be predetermined so as to achieve desired TCR characteristics.
These as well as other aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments.